A bar resistor is typically formed by a rectangular, resistive layer which is deposited on an insulating substrate and is contacted on each of two opposite edges by a conductive terminal. The resistive value of such a resistor depends on the surface geometry of the resistive layer between its two terminals, and on the resistivity of the layer, such resistivity being typically characterized in ohms per square unit of film area, which is simply designated as ohms per square.
Because of variations in the resistivity of the resistive layers, bar resistors, by design, typically are formed to have values well below the desired values of the resistors. The desired values of the resistors are thereafter attained by altering the straight electrical paths through the resistive layers between the terminals. The process of altering the resistors is known as resistor trimming.
The electric paths between the terminals are typically altered by one or both of two different, yet related techniques. By one technique, a path is simply narrowed, such that, in relation to its width, the path is lengthened whereby its resistance is increased. According to a second technique, staggered cuts are made into alternately opposite edges of a resistive layer to change an originally straight electrical path into a serpentine path. These cuts, referred to as plunge cuts, also lengthen the electrical path between the terminals of the respective resistor and, hence, raise its resistance.
The state of the art for making such resistor trim cuts is well advanced to the extent that commercially available systems using laser energy for the trim cuts typically use commercially available mini or micro computers to direct the laser cutting operations. Such commercially available systems as, for example, one available from Teradyne, Inc. permit laser cuts to continue for predetermined distances, using coordinate inputs, or to stop a cut when a resistor which is being trimmed reaches a certain resistive value. A resistance bridge circuit is used for measuring the resistance of the resistor being trimmed. Typically the measurement is not continuous but is made between laser pulses in discrete measuring steps at the pulse rate of the laser.
An existing control program furnished with such a commercially available system also permits the laser cut to be redirected to continue from the end of a first cut with a second cut at right angles to the first cut, when such first cut has reached a preset value. This latter program therefore permits a combination of a plunge cut and a longitudinal cut as a final trim cut.
The cuts are terminated when the preset position of the laser is reached, or as soon as the resistance bridge indicates that a desired value is reached, the next subsequent laser pulse is inhibited.
In spite of the availability of the described automated trimming apparatus, it has nevertheless been difficult to trim film resistors to value with a high degree of precision. For example, when trimmed resistor values are required to fall within a tolerance range of plus or minus one-tenth of one percent, typical resistor trim techniques were found too inaccurate to be used at reasonably economical speeds. A technique for trimming resistors at reasonable speeds and to a relatively high degree of accuracy is, therefore, desirable.